Discovery and In Situ Crystallization Studies of Cerium-Based Metal–Organic Frameworks with V-Shaped Linker Molecules

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We report the discovery and characterization of two porous Ce(III)-based metal–organic frameworks (MOFs) with the V-shaped linker molecules 4,4′-sulfonyldibenzoate (SDB 2– ) and 4,4′-(hexafluoroisopropylidene)bis(benzoate) (hfipbb 2– ). The compounds of framework composition [Ce 2 (H 2 O)(SDB) 3 ] ( 1 ) and [Ce 2 (hfipbb) 3 ] ( 2 ) were obtained by using a synthetic approach in acetonitrile that we recently established. Structure determination of 1 was accomplished from 3D electron diffraction (3D ED) data, while 2 could be refined against powder X-ray diffraction (PXRD) data using the crystal structure of an isostructural La-MOF as the starting model. Their framework structures consist of chain-like inorganic building units (IBUs) or hybrid-BUs that are interconnected by the V-shaped linker molecules to form framework structures with channel-type pores. The composition of both compounds was confirmed by PXRD, elemental analysis, as well as NMR and IR spectroscopy. Interestingly, despite the use of (NH 4 ) 2 [Ce IV (NO 3 ) 6 ] in the synthesis, cerium ions in both MOFs occur exclusively in the + III oxidation state as determined by X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS). Thermal analyses reveal remarkably high thermal stabilities of ≥400 °C for the MOFs. Initial N 2 sorption measurements revealed the peculiar sorption behavior of 2 which prompted a deeper investigation by Ar and CO 2 sorption experiments. The combination with nonlocal density functional theory (NL-DFT) calculations adds to the understanding of the nature of the different pore diameters in 2 . An extensive quasi-simultaneous in situ XANES/XRD investigation was carried out to unveil the formation of Ce-MOFs during the solvothermal syntheses in acetonitrile. The crystallization of the two Ce(III)-MOFs presented herein as well as two previously reported Ce(IV)-MOFs, all obtained by a similar synthetic approach, were studied. While the XRD patterns show time-dependent MOF crystallization, the XANES data reveal the presence of Ce(III) intermediates and their subsequent conversion to the MOFs. The addition of acetic acid in combination with the V-shaped linker molecule was identified as the crucial factor for the formation of the crystalline Ce(III/IV)-MOFs.

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